According to the laws of thermodynamics, the heat equivalent of the mechanical work produced is the difference between the heat supplied to the system and the heat leaving the system at the end of its thermodynamic cycle. For maximum efficiency, the mechanical work generated must be as high as possible and the thermo efficiency of the system is therefore given by the formula, Efficiency=(Q1−Q2)/Q1 where Q1 is the heat supplied and Q2 is the heat leaving the system. This equation can then be written in the form Thermal Efficiency=(1−Q2/Q1).
Engines where heat is added by conduction through an external heat exchanger normally works on the Stirling cycle, which was invented by Robert Stirling in 1816. These engines work on the principle of having a given mass of gas inside a closed chamber with a displacer piston offset in a cyclic motion from the main power piston. The displacer piston pushes the air alternatively between a hot source, where heat is added thus increasing temperature and pressure and a cold sink where heat is removed by cooling thereby reducing temperature and pressure. The power piston is thereby driven by a pulsating pressure wave. The advantage of these engines being the freedom to operate by any type of fuel, it's complete freedom from toxic emissions and quiet operations and has been the preferred prime mover in Combined Heat and Power (CHP) applications.
In heat engines producing mechanical work only, the heat leaving the system is wasted, and the efficiency of mechanical work produced is therefore the only measure of fuel efficiency which should produce mechanical work with the minimum of fuel consumed.
In the fast growing and increasingly important segment of CHP for domestic and other application, the heat leaving the system can be used to heat domestic hot water and heating and therefore makes better use of the total energy supplied and can typically achieve total efficiencies of 80-90% of the heat supplied. Typically, the electric power demand for a domestic CHP unit is between 3-5 kW of power and be expected to operate continuously, with normal service demands only, for a minimum period of 5 years.
Heat engines which are based on the above described principle may for example be found in U.S. Pat. No. 6,672,063; U.S. Pat. No. 1,926,463; DE 23 36 105; DE 10 2006 039 924; GB 127,686; US 2006/218 919; DE 10 2005 053 857; U.S. Pat. No. 4,077,221; U.S. Pat. No. 3,879,945; JP 2011-038 508 and DE 2 204 763.
It is further known to use dual cylinder chamber engines in which a gas flow is directed first through a first chamber defined at a first end of the cylinder where after the gas is directed to a second chamber defined at the other end of the cylinder wherein heat is added to the gas flow in a heater outside the cylinder. This is for example shown in U.S. Pat. No. 2,067,453; U.S. Pat. No. 2,685,173 and U.S. Pat. No. 3,138,918
Even though the above described systems describes working examples of suitable engines and control methods using the Stirling cycle, there is still a desire for improvements in the constructional details of the engine in order to reduce losses and optimize the engine performance while also providing an engine which may work continuously for a long period with low wear of the components.